Assay for RCR. PCR was performed on peripheral blood leukocyte DNA obtained from mice with a myeloproliferative disorder. DNA from an infected monkey 15445 served as a positive control for PCR and demonstrates the sensitivity of the assay (upper right). For test samples, 200 ng of template DNA was used for PCR with either oligos specific for RCV or for β-globin that served as an internal control. No helper genome was detectable in all samples tested. At the time of analysis all mice had elevated leukocyte counts except for MDR11. C.R. no. 1 is from the competitive repopulation experiment 1 (MDR1 vfresh marrow). MDR20 nos. 1, 4, 5, and 6 represent secondary transplanted mice from MDR20.